Conventionally, in dust respirators, various types of air conditioning elements, air cleaners, cabin filters, and various types of apparatuses, porous filters have been used for purposes of dust collection, protection, ventilation, and the like.
Among the porous filters, filters made of fibrous materials have high porosity and have advantages such as long life and low ventilation resistance, and have therefore been widely used. These filters made of fibrous materials collect particles on fibers by mechanical collecting mechanisms such as blocking, diffusion, and inertial impaction, and have been known to have the minimum value of collection efficiency of the filter when the particles have an aerodynamic equivalent particle diameter of about 0.1 to 1.0 μm in practical application environments.
To improve the collection efficiency of the filter at the above-mentioned minimum value, a method of using electric attraction in combination is known. For example, a method employed may be a method for imparting an electric charge to object particles to be collected, a method for imparting an electric charge to a filter, or a combination of both of them. For examples of the method for imparting an electrostatic charge to a filter, there is known a method of disposing a filter between electrodes and causing dielectric polarization at the time of ventilation, and a method for imparting a long life electrostatic charge to an insulating material. Particularly, the latter technique has widely been employed for an electret filter since there is no need to use energy such as an external power source.
For electret filters, to increase initial collection efficiency and also to suppress performance decrease due to attenuation of electrostatic charge at the time of processing or preserving filters, electret materials capable of being electretized and excellent in moisture-proof stability and heat-resistant stability are used.
However, electret filters have a disadvantage of decrease of electrostatic attraction caused by collection of particles, and especially, oil mist that has low surface tension markedly accelerates elimination of electric charge by thinly coating the fiber surfaces. For common electret filters, polyolefins, polyesters, polycarbonates, phenol resins, and the like, that are excellent in electric charge stability are used. However, even fibrous materials of polyolefins such as polypropylene, polyethylene, and polymethylpentene with smallest surface tension among these materials do not exhibit sufficient oil repellency as a material characteristic against oil mist represented by poly-α-olefins (PAO), dioctyl phthalate (DOP), tobacco smoke, and the like. Therefore, there is a problem that collection efficiency retention capacity at the time the oil mist is loaded (hereinafter, referred to as oil mist resistance) is low.
To solve such a problem, there is known a method of imparting oil repellency to the filter by lowering surface tension of a fibrous material composing the filter, and improving the oil mist resistance by suppressing spread of mist on the fiber surface and absorption and diffusion of mist in the inside of the fibrous material and thereby decreasing elimination of electric charge. Specifically, electrets employed are those having lowered surface tension and heightened oil mist resistance while retaining electric charge stability by a method of mixing an additive having a perfluoro group to the inside of resin to increase oil repellency (for example, Patent Document 1), a method of melt-spinning a thermoplastic fluororesin (for example, Patent Document 2 and Patent Document 3), a method of coating a surface with an emulsion processing agent containing a perfluoro group (for example, Patent Document 4), a method of introducing fluorine atoms by replacing hydrogen atoms using plasma, a fluorine gas and the like (for example, Patent Document 5), and the like.
Hereinafter, lowering surface tension of a material will be described as “oil repellency,” and an effect of suppressing decrease in efficiency against oil mist will be described as “oil mist resistance.” The oil repellency mentioned in the present invention means the effect of suppressing spread of a liquid by lowering surface tension, and in consideration of the principle of wetting, the oil repellency also encompasses the action against water with high surface tension value (water repellency).
However, a fluorine-based resin or a fluorine-based low molecular weight additive is unsuitable for melt spinning, since dissociation of fluoro-telomers and production of hydrogen fluoride and carbonyl fluoride as thermal decomposition products occur in environments exceeding 320° C. In addition, in the case of fluorine atom introduction by fluorine gas or a plasma treatment, in order to prevent fluorine gas leakage and suppress hydrophilization, it is necessary to strictly control the oxygen and water amounts, and special facilities with high air tightness are accordingly required. Further, because of an issue of bioaccumulation, use of PFOA (perfluorooctanoic acid), PFOS (perfluorooctanesulfonic acid) and salts thereof, as well as use of mother materials for producing telomers and production of the materials are inhibited. Thus, processes in which these materials are added, fluorine-hydrogen exchange randomly occurs, or thermal decomposition or oxidation decomposition occurs are not preferable.
Further, a fluorine-containing acrylate-based processing agent, which is developed for textiles, contains an emulsifier or a film-forming auxiliary, and includes a short chain perfluoro group equal to or shorter than C6F13 as a side chain to comply with PFOA and PFOS regulations and therefore, the processing agent loses crystallinity. Thus, the processing agent itself has a problem that not only the agent does not have the stability of electrostatic charge but also the agent considerably inhibits electric charge stability of a fibrous material serving as a substrate even for a small amount of deposition.
Further, also known are fluorine-based resins which have solubility and thermoplasticity by being made amorphous, and have both electric charge stability and coating suitability (for example, Patent Document 6). However, these resins have a problem that it is required to use a special monomer as a main skeleton, and the production cost significantly increases accordingly.
Still further, methods of adding various kinds of additives to improve heat resistant stability of electret materials are disclosed (for example, Patent Document 7), and there are also known methods of increasing the electric charge quantity at the time of liquid contact by mixing an electric charge-enhancing additive to improve the electrostatic charge quantity and thus improve the collection efficiency of the filter (for example, Patent Document 8).
However, the inventors of the present invention made investigations and confirmed the following problems: addition of an electric charge-enhancing additive increases the surface tension of an electret material, especially, collection of oil mist with low surface tension and thin coating of the fiber surface with the oil mist considerably accelerate elimination of electric charge.
That is, these electret filters fail to exhibit sufficient oil repellency as a material characteristic against oil mist represented by various kinds of mineral oils, plant oils, poly-α-olefins (PAO), dioctyl phthalate (DOP), tobacco smoke, or the like, and accordingly have a problem of low oil mist resistance and decrease of average collection efficiency although having high initial collection efficiency.